System and method for variable compression ratio engine

ABSTRACT

Methods and systems are provided for improving calibration of a variable compression ratio engine. Cylinder-to-cylinder compression ratio variations are detected and accounted for by comparing cylinder fuel flow and IMEP at each compression ratio setting. Dilution parameters including EGR and VCT schedule are also calibrated to account for the cylinder-to-cylinder compression ratio variations.

FIELD

The present description relates generally to methods and systems forcontrolling a compression ratio of a variable compression ratio engine.

BACKGROUND/SUMMARY

The compression ratio (CR) of an internal combustion engine is definedas the ratio of the cylinder volume when the piston is atbottom-dead-center (BDC) to the cylinder volume when the piston is attop-dead-center (TDC). Generally, the higher the compression ratio, thehigher the thermal efficiency and fuel economy of the internalcombustion engine. Variable Compression Ratio (VCR) engines have beendeveloped wherein the compression ratio of each cylinder can be variedbetween a higher and a lower setting to improve engine performance. Forexample, the higher compression ratio setting may be used duringknock-free conditions to take advantage of the high thermal efficiencywhile the lower compression ratio setting may be used during knock proneconditions. In the VCR engines, a linkage or other mechanism (e.g., aneccentric) may be coupled to the piston of each cylinder to mechanicallyvary the compression ratio between the higher and lower settings.

One example of a VCR engine is shown by Caswell at U.S. Pat. No.4,469,055. Therein, during engine running, the CR of the engine isadjusted based on engine operating conditions. For example, the CR maybe optimized for engine fuel efficiency or engine performance, or both.The CR calibration, that is the CR commanded as a function of enginespeed and load, may be calibrated based on a prototype engine.

However, the inventors herein have identified potential issues with suchsystems. As one example, the adjustment of the CR during engineoperation requires the actual CR to be known accurately. However, eachengine may have a slightly different compression ratio (CR) in eachcylinder, due to manufacturing tolerances. In a VCR engine, eachcomponent of the VCR mechanism may have manufacturing tolerances leadingto significant part-to-part variation, in addition to the normalvariation on non-VCR engines. A VCR calibration based on the average CR(that is, the average of the CR across all engine cylinders) may resultin extra spark retard usage on those cylinders which have ahigher-than-average CR, leading to a much lower efficiency on thosecylinders. Use of premium manufacturing methods and/or “select fit”parts can be used to control CR differences between cylinders, but suchapproaches add significant cost. Since VCR engines raise the compressionratio as much as possible, they tend to be knock-limited over a largeportion of the engine operating map, and for a large fraction of a drivecycle. Without knowing the actual CR of each cylinder, and thecylinder-to-cylinder variations, it may be difficult to optimize the CRcalibration, resulting in engine performance losses.

In one example, the above issues may be at least partly addressed by amethod comprising: actuating a variable compression ratio mechanism ofan engine to mechanically adjust a target compression ratio of theengine in accordance with an updated calibration, the updatedcalibration based on each of fuel flow and peak torque of each cylinderat each compression ratio setting of the mechanism. In this way, CRoptimization of a VCR engine is improved.

As one example, the actual CR of each cylinder of a VCR engine may bequantified as a function of each VCR mechanism setting. For example, theCR of each engine cylinder may be quantified first while operating theVCR engine at a lower CR setting. Then, the CR of each engine cylindermay be quantified while operating the VCR engine at a higher CR setting.Then, the fuel flow and maximum IMEP of each cylinder may be quantifiedas a function of each VCR mechanism setting. Further, the parameters maybe quantified as a function of the existing engine operating conditions,such as engine speed, engine torque, fuel octane, inlet air temperature,humidity, etc. The fuel flow and IMEP of all cylinders may then besummed to quantify the total engine fuel flow and total IMEP of theengine as a function of each VCR mechanism setting, at the currentoperating conditions. Thereafter, at each engine operating conditionwhere driver demand is below a threshold, the engine controller mayselect the VCR mechanism setting which gives the minimum total enginefuel flow. At each operating condition where driver demand is above thethreshold, the controller may select the VCR mechanism setting whichgives the maximum total engine IMEP. The threshold may be apre-determined value, or it may be adjusted as a function of currentengine speed, fuel octane, ambient temperature, humidity, etc.

In this way, the efficiency of a VCR engine may be improved by detectingand compensating for cylinder-to-cylinder variations in compressionratio. The technical effect of learning fuel flow and IMEP of allcylinders as a function of each CR setting of the VCR engine is that CRvariations of the actual engine may be learned, instead of relying on aprototype engine which may be significantly different from the givenengine. Further, the VCR engine can be calibrated without relying onexpensive manufacturing methods and/or components. By selecting a CRsetting for the VCR engine that corresponds to the lowest total enginefuel flow when operator torque demand is low, fuel consumption andcarbon dioxide (CO2) emissions can be minimized. By selecting a CRsetting for the VCR engine that corresponds to the highest total torquewhen operator torque demand is high, engine performance can bemaximized. Overall, engine performance and fuel efficiency of a VCRengine can be improved.

It should be understood that the summary above is provided to introducein simplified form a selection of concepts that are further described inthe detailed description. It is not meant to identify key or essentialfeatures of the claimed subject matter, the scope of which is defineduniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example variable compression ratio (VCR) engine system.

FIG. 2 depicts an example high-level flow chart for optimizing the CRcalibration and dilution calibration of a VCR engine.

FIG. 3 depicts another example high-level flow chart for optimizing theCR calibration and dilution calibration of a VCR engine.

FIG. 4 depicts an example table showing differences between actual andexpected CR of a VCR engine.

FIG. 5 depicts example differences between a nominal and a modified EGRand VCT schedule of a VCR engine.

FIG. 6 depicts a prophetic example of VCR engine control.

FIG. 7 depicts example differences between a nominal and a modified CRschedule of a VCR engine.

DETAILED DESCRIPTION

The following description relates to systems and methods for enginesystem configured with a variable compression ratio (VCR) mechanism, asdescribed with reference to the engine system of FIG. 1. A controllermay be configured to perform a control routine, such as the exampleroutine of FIGS. 2-3, to calibrate the CR commanded at a given enginespeed-load by learning actual cylinder-to-cylinder variations in CRbased on differences in fuel economy and output torque of each cylinderat each CR setting of the VCR engine. During conditions when dilutioncontrol is required, the controller may also modify a nominal EGR(Exhaust Gas Recirculation) or VCT (Variable Camshaft Timing) schedulebased on the mapped cylinder-to-cylinder CR variations. Examplemodifications to a CR calibration and an EGR calibration are shown atthe tables of FIGS. 4-5 and 7. An example of adjusting engine operationbased on the CR and EGR calibration is shown with reference to FIG. 6.In this way, the performance and fuel economy of a VCR engine can beimproved.

FIG. 1 depicts an example embodiment of a combustion chamber or cylinderof internal combustion engine 10. Engine 10 may be included in a vehiclesystem 5, such as a vehicle configured for on-road propulsion. Engine 10may receive control parameters from a control system includingcontroller 12 and input from a vehicle operator 130 via an input device132. In this example, input device 132 includes an accelerator pedal anda pedal position sensor 134 for generating a proportional pedal positionsignal PP. Cylinder (herein also “combustion chamber”) 14 of engine 10may include combustion chamber walls 136 with piston 138 positionedtherein. Piston 138 may be coupled to crankshaft 140 so thatreciprocating motion of the piston is translated into rotational motionof the crankshaft. Crankshaft 140 may be coupled to at least one drivewheel of the passenger vehicle via a transmission system. Further, astarter motor may be coupled to crankshaft 140 via a flywheel to enablea starting operation of engine 10.

Engine 10 may be configured as a variable compression ratio (VCR) enginewherein the compression ratio (CR) of each cylinder (that is, the ratioof the cylinder volume when the piston is at bottom-dead-center (BDC) tothe cylinder volume when the piston is at top-dead-center (TDC)) can bemechanically altered. The CR of the engine may be varied via a VCRactuator 192 actuating a VCR mechanism 194. In some example embodiments,the CR may be varied between a first, lower CR (wherein the ratio ofcylinder volume when the piston is at BDC to the cylinder volume whenthe piston is at TDC is smaller) and a second, higher CR (wherein theratio is higher). In still other example embodiments, there may bepredefined number of stepped compression ratios. Further still, the CRmay be continuously variable between the first, lower CR and the second,higher CR (to any CR in between).

In the depicted example, VCR mechanism 194 is coupled to piston 138 suchthat the VCR mechanism may change the piston TDC position. For example,piston 138 may be coupled to crankshaft 140 via a piston positionchanging VCR mechanism 194 that moves the pistons closer to or furtherfrom the cylinder head, thus changing the size of combustion chamber 14.A position sensor 196 may be coupled to the VCR mechanism 192 and may beconfigured to provide feedback to controller 12 regarding the positionof VCR mechanism 194 (and thereby the compression ratio) being appliedto the cylinder.

In one example, changing the position of the piston within thecombustion chamber also changes the relative displacement of the pistonwithin the cylinder. The piston position changing VCR mechanism may becoupled to a conventional cranktrain or an unconventional cranktrain.Non-limiting examples of an unconventional cranktrain to which the VCRmechanism may be coupled include variable distance head crankshafts andvariable kinematic length crankshafts. In one example, crankshaft 140may be configured as an eccentric shaft. In another example, aneccentric may be coupled to, or in the area of a piston pin, theeccentric changing the position of the piston within the combustionchamber. Movement of the eccentric may be controlled by oil passages inthe piston rod.

It will be appreciated that still other VCR mechanisms that mechanicallyalter the compression ratio may be used. For example, the CR of theengine may be varied via a VCR mechanism that changes a cylinder headvolume (that is, the clearance volume in the cylinder head). In stillanother example, the VCR mechanism may include a hydraulic pressure, airpressure, or mechanical spring reactive piston. Further still, the VCRmechanism may include a multi-link mechanism or a bent rod mechanism.Still other VCR mechanizations may be possible. It will be appreciatedthat as used herein, the VCR engine may be configured to adjust the CRof the engine via mechanical adjustments that vary a piston position ora cylinder head position or a cylinder head volume. As such, VCRmechanisms do not include effective CR adjustments achieved viaadjustments to a valve timing or cam timing.

By adjusting the position of the piston within the cylinder, an actual(static) compression ratio of the engine (that is a difference betweencylinder volumes at TDC relative to BDC) can be varied. In one example,reducing the compression ratio includes reducing a displacement of thepiston within the combustion chamber by increasing a distance between atop of the piston from a cylinder head. For example, the engine may beoperated at a first, lower compression ratio by the controller sending asignal to actuate the VCR mechanism to a first position where the pistonhas a smaller effective displacement within the combustion chamber. Asanother example, the engine may be operated at a second, highercompression ratio by the controller sending a signal to actuate the VCRmechanism to a second position where the piston has a larger effectivedisplacement within the combustion chamber. Changes in the enginecompression ratio may be advantageously used to improve fuel economy.For example, a higher compression ratio may be used to improve fueleconomy at light to moderate engine loads until spark retard from earlyknock onset erodes the fuel economy benefit. The engine can then beswitched to a lower compression ratio, thereby trading off theefficiency benefits of higher compression ratio for the efficiencybenefits of optimized combustion phasing. Continuous VCR systems maycontinuously optimize the trade-offs between combustion phasing and theefficiency benefits of higher compression ratio, to provide the bestcompression ratio between the higher compression ratio and lowercompression ratio limits at the given operating conditions. In oneexample, an engine controller may refer a look-up table to select acompression ratio to apply based on engine speed-load conditions. Aselaborated below, the selecting may include selecting a lowercompression ratio at higher engine loads, and selecting a highercompression ratio at lower engine loads.

Cylinder 14 can receive intake air via a series of intake air passages142, 144, and 146. Intake air passage 146 can communicate with othercylinders of engine 10 in addition to cylinder 14. In some embodiments,one or more of the intake passages may include a boosting device such asa turbocharger or a supercharger. For example, FIG. 1 shows engine 10configured with a turbocharger including a compressor 174 arrangedbetween intake passages 142 and 144, and an exhaust turbine 176 arrangedalong exhaust passage 148. Compressor 174 may be at least partiallypowered by exhaust turbine 176 via a shaft 180 where the boosting deviceis configured as a turbocharger. However, in other examples, such aswhere engine 10 is provided with a supercharger, exhaust turbine 176 maybe optionally omitted, where compressor 174 may be powered by mechanicalinput from a motor of the engine. A throttle 20 including a throttleplate 164 may be provided along an intake passage of the engine forvarying the flow rate and/or pressure of intake air provided to theengine cylinders. For example, throttle 20 may be disposed downstream ofcompressor 174 as shown in FIG. 1, or alternatively may be providedupstream of compressor 174.

Exhaust passage 148 can receive exhaust gases from other cylinders ofengine 10 in addition to cylinder 14. Exhaust gas sensor 128 is showncoupled to exhaust passage 148 upstream of emission control device 178.Sensor 128 may be selected from among various suitable sensors forproviding an indication of exhaust gas air/fuel ratio such as a linearoxygen sensor or UEGO (universal or wide-range exhaust gas oxygen), atwo-state oxygen sensor or EGO (as depicted), a HEGO (heated EGO), aNOx, HC, or CO sensor, for example. Emission control device 178 may be athree way catalyst (TWC), NOx trap, various other emission controldevices, or combinations thereof.

Exhaust temperature may be estimated by one or more temperature sensors(not shown) located in exhaust passage 148. Alternatively, exhausttemperature may be inferred based on engine operating conditions such asspeed, load, air-fuel ratio (AFR), spark retard, etc. Further, exhausttemperature may be computed by one or more exhaust gas sensors 128. Itmay be appreciated that the exhaust gas temperature may alternatively beestimated by any combination of temperature estimation methods listedherein.

Each cylinder of engine 10 may include one or more intake valves and oneor more exhaust valves. For example, cylinder 14 is shown including atleast one intake poppet valve 150 and at least one exhaust poppet valve156 located at an upper region of cylinder 14. In some embodiments, eachcylinder of engine 10, including cylinder 14, may include at least twointake poppet valves and at least two exhaust poppet valves located atan upper region of the cylinder.

Intake valve 150 may be controlled by controller 12 by cam actuation viacam actuation system 151. Similarly, exhaust valve 156 may be controlledby controller 12 via cam actuation system 153. Cam actuation systems 151and 153 may each include one or more cams and may utilize one or more ofcam profile switching (CPS), variable cam timing (VCT), variable valvetiming (VVT) and/or variable valve lift (VVL) systems that may beoperated by controller 12 to vary valve operation. The position ofintake valve 150 and exhaust valve 156 may be determined by valveposition sensors 155 and 157, respectively. In alternative embodiments,the intake and/or exhaust valve may be controlled by electric valveactuation. For example, cylinder 14 may alternatively include an intakevalve controlled via electric valve actuation and an exhaust valvecontrolled via cam actuation including CPS and/or VCT systems. In stillother embodiments, the intake and exhaust valves may be controlled by acommon valve actuator or actuation system, or a variable valve timingactuator or actuation system.

Cylinder 14 can have a compression ratio, which is the ratio of volumeswhen piston 138 is at bottom center to top center. Conventionally, thecompression ratio is in the range of 9:1 to 10:1. However, in someexamples where different fuels are used, the compression ratio may beincreased. This may happen, for example, when higher octane fuels orfuels with higher latent enthalpy of vaporization are used. Thecompression ratio may also be increased if direct injection is used dueto its effect on engine knock. The compression ratio may also bemechanically varied based on driver demand via adjustments to a VCRactuator 192 that actuates a VCR mechanism 194, varying the effectiveposition of piston 138 within combustion chamber 14. The compressionratio may be inferred based on feedback from sensor 196 regarding theposition of the VCR mechanism 194.

In some embodiments, each cylinder of engine 10 may include a spark plug192 for initiating combustion. Ignition system 190 can provide anignition spark to combustion chamber 14 via spark plug 192 in responseto spark advance signal SA from controller 12, under select operatingmodes. However, in some embodiments, spark plug 192 may be omitted, suchas where engine 10 may initiate combustion by auto-ignition or byinjection of fuel as may be the case with some diesel engines.

In some embodiments, each cylinder of engine 10 may be configured withone or more fuel injectors for providing fuel thereto. As a non-limitingexample, cylinder 14 is shown including one fuel injector 166. Fuelinjector 166 is shown coupled directly to cylinder 14 for injecting fueldirectly therein in proportion to the pulse width of signal FPW receivedfrom controller 12 via electronic driver 168. In this manner, fuelinjector 166 provides what is known as direct injection (hereafter alsoreferred to as “DI”) of fuel into combustion cylinder 14. While FIG. 1shows injector 166 as a side injector, it may also be located overheadof the piston, such as near the position of spark plug 192. Such aposition may improve mixing and combustion when operating the enginewith an alcohol-based fuel due to the lower volatility of somealcohol-based fuels. Alternatively, the injector may be located overheadand near the intake valve to improve mixing. Fuel may be delivered tofuel injector 166 from a high pressure fuel system 8 including fueltanks, fuel pumps, and a fuel rail. Alternatively, fuel may be deliveredby a single stage fuel pump at lower pressure, in which case the timingof the direct fuel injection may be more limited during the compressionstroke than if a high pressure fuel system is used. Further, while notshown, the fuel tanks may have a pressure transducer providing a signalto controller 12. It will be appreciated that, in an alternateembodiment, injector 166 may be a port injector providing fuel into theintake port upstream of cylinder 14.

It will also be appreciated that while the depicted embodimentillustrates the engine being operated by injecting fuel via a singledirect injector; in alternate embodiments, the engine may be operated byusing two or more injectors (for example, a direct injector and a portinjector per cylinder, or two direct injectors/two port injectors percylinder, etc.) and varying a relative amount of injection into thecylinder from each injector.

Fuel may be delivered by the injector to the cylinder during a singlecycle of the cylinder. Further, the distribution and/or relative amountof fuel delivered from the injector may vary with operating conditions.Furthermore, for a single combustion event, multiple injections of thedelivered fuel may be performed per cycle. The multiple injections maybe performed during the compression stroke, intake stroke, or anyappropriate combination thereof. Also, fuel may be injected during thecycle to adjust the air-to-injected fuel ratio (AFR) of the combustion.For example, fuel may be injected to provide a stoichiometric AFR. AnAFR sensor may be included to provide an estimate of the in-cylinderAFR. In one example, the AFR sensor may be an exhaust gas sensor, suchas EGO sensor 128. By measuring an amount of residual oxygen (for leanmixtures) or unburned hydrocarbons (for rich mixtures) in the exhaustgas, the sensor may determine the AFR. As such, the AFR may be providedas a Lambda (λ) value, that is, as a ratio of actual AFR tostoichiometry for a given mixture. Thus, a Lambda of 1.0 indicates astoichiometric mixture, richer than stoichiometry mixtures may have alambda value less than 1.0, and leaner than stoichiometry mixtures mayhave a lambda value greater than 1.

As described above, FIG. 1 shows only one cylinder of a multi-cylinderengine. As such each cylinder may similarly include its own set ofintake/exhaust valves, fuel injector(s), spark plug, etc.

Fuel tanks in fuel system 8 may hold fuel with different fuel qualities,such as different fuel compositions. These differences may includedifferent alcohol content, different octane, different heat ofvaporizations, different fuel blends, and/or combinations thereof etc.

Engine 10 may further include a knock sensor 90 coupled to each cylinder14 for identifying abnormal cylinder combustion events. In alternateembodiments, one or more knock sensors 90 may be coupled to selectedlocations of the engine block. The knock sensor may be an accelerometeron the cylinder block, or an ionization sensor configured in the sparkplug of each cylinder. The output of the knock sensor may be combinedwith the output of a crankshaft acceleration sensor to indicate anabnormal combustion event in the cylinder. In one example, based on theoutput of knock sensor 90 in one or more defined windows (e.g., crankangle timing windows), abnormal combustion due to one or more of knockand pre-ignition may be identified and differentiated. For example,knock may be identified responsive to knock sensor output estimated in aknock window being higher than a knock threshold, while pre-ignition maybe identified responsive to knock sensor output estimated in apre-ignition window being higher than a pre-ignition threshold, thepre-ignition threshold higher than the knock threshold, the pre-ignitionwindow earlier than the knock window. Further, the abnormal combustionmay be accordingly addressed. For example, knock may be addressed byreducing the compression ratio and/or retarding spark timing whilepre-ignition may be addressed by enriching the engine and/or limiting anengine load. In addition, lowering the compression ratio also reducesthe changes of further pre-ignition.

Controller 12 is shown as a microcomputer, including microprocessor unit106, input/output ports 108, an electronic storage medium for executableprograms and calibration values shown as read only memory chip 110 inthis particular example, random access memory 112, keep alive memory114, and a data bus. Controller 12 may receive various signals fromsensors coupled to engine 10, in addition to those signals previouslydiscussed, including measurement of inducted mass air flow (MAF) frommass air flow sensor 122; engine coolant temperature (ECT) fromtemperature sensor 116 coupled to cooling sleeve 118; a profile ignitionpickup signal (PIP) from Hall effect sensor 120 (or other type) coupledto crankshaft 140; throttle position (TP) from a throttle positionsensor; absolute manifold pressure signal (MAP) from sensor 124,cylinder AFR from EGO sensor 128, abnormal combustion from knock sensor90 and a crankshaft acceleration sensor, and VCR mechanism position fromposition sensor 196. Engine speed signal, RPM, may be generated bycontroller 12 from signal PIP. Manifold pressure signal MAP from amanifold pressure sensor may be used to provide an indication of vacuum,or pressure, in the intake manifold. The controller 12 receives signalsfrom the various sensors of FIG. 1 and employs the various actuators ofFIG. 1 to adjust engine operation based on the received signals andinstructions stored on a memory of the controller. For example, based onthe engine speed and load, the controller may adjust the compressionratio of the engine by sending a signal to the VCR actuator whichactuates the VCR mechanism to mechanically move the piston closer to orfurther from the cylinder head, to thereby change a volume of thecombustion chamber.

Non-transitory storage medium read-only memory 110 can be programmedwith computer readable data representing instructions executable byprocessor 106 for performing the methods described below as well asother variants that are anticipated but not specifically listed.

In some examples, vehicle 5 may be a hybrid vehicle with multiplesources of torque available to one or more vehicle wheels 55. In otherexamples, vehicle 5 is a conventional vehicle with only an engine or anelectric vehicle with only an electric machine(s). In the example shown,vehicle 5 includes engine 10 and an electric machine 52. Electricmachine 52 may be a motor or a motor/generator. Crankshaft 140 of engine10 and electric machine 52 are connected via transmission 54 to vehiclewheels 55 when one or more clutches 56 are engaged. In the depictedexample, a first clutch 56 is provided between crankshaft 140 andelectric machine 52, and a second clutch 56 is provided between electricmachine 52 and transmission 54. Controller 12 may send a signal to anactuator of each clutch 56 to engage or disengage the clutch, so as toconnect or disconnect crankshaft 140 from electric machine 52 and thecomponents connected thereto, and/or connect or disconnect electricmachine 52 from transmission 54 and the components connected thereto.Transmission 54 may be a gearbox, a planetary gear system, or anothertype of transmission. The powertrain may be configured in variousmanners including as a parallel, a series, or a series-parallel hybridvehicle.

Electric machine 52 receives electrical power from a traction battery 58to provide torque to vehicle wheels 55. Electric machine 52 may also beoperated as a generator to provide electrical power to charge battery58, for example, during a braking operation.

The actual CR of each cylinder affects the knock limit of that cylinder,particularly at high loads, as well as the dilution limits for thatcylinder, particularly at light loads. Due to manufacturing tolerancesin the VCR mechanism coupled to each cylinder 30 of engine 10, there maybe significant part-to-part variation between the actual CR of eachcylinder and the expected CR for that cylinder. In addition, for a givenexpected CR, there may be significant cylinder-to-cylinder variation inactual CR. As a result of these differences, CR calibration may benon-optimal. Since the CR of an engine also affects the engine'sdilution tolerance, errors in CR estimation can also result innon-optimal EGR or VCT (or VVL etc.) calibration. As one example, alower CR setting may be commanded responsive to high load conditions.However, due to the actual CR of a cylinder being higher than expected,the resulting non-optimal CR may be higher than desired, resulting inthe cylinder becoming excessively knock limited. As another example, ahigher CR setting may be commanded responsive to low load conditions.However, due to the actual CR of a cylinder being lower than expected,the resulting non-optimal CR may be lower than desired, resulting in thecylinder becoming combustion stability and NVH limited.

As elaborated with reference to FIGS. 2-3, an engine controller mayupdate a CR calibration (that is, a calibration of the CR to command ata given engine speed and load) based on calculated differences in fuelusage and torque output of each cylinder at each CR setting of the VCRengine. The engine controller may also update an EGR and/or VCTcalibration (that is, a calibration of the dilution to command at agiven engine speed and load) based on the calculated differences in fuelusage and torque output of each cylinder at each CR setting of the VCRengine. As a result, VCR engine performance may be improved.

Now turning to FIG. 2, an example routine 200 is described forcalibrating a VCR engine. The method reduces performance losses arisingfrom cylinder-to-cylinder variations in CR, due to manufacturingtolerances. Instructions for carrying out method 200 as well the othermethods included herein may be executed by a controller based oninstructions stored on a memory of the controller and in conjunctionwith signals received from sensors of the engine system, such as thesensors described above with reference to FIG. 1. The controller mayemploy engine actuators of the engine system to adjust engine operation,according to the methods described below.

At 202, method 200 includes estimating and/or measuring engine operatingconditions. Engine operating conditions may include, for example, driverpower demand (for example, as based on an output of a pedal positionsensor coupled to an operator pedal); ambient temperature, pressure, andhumidity; engine speed, engine temperature; manifold pressure (MAP);manifold air flow (MAF); catalyst temperature; intake temperature; boostlevel; fuel octane of fuel available in a fuel tank; etc.

It will be appreciated that in an alternate example, method 200 may betriggered during a first engine start following manufacture of theengine to allow for the engine to be calibrated. In still otherexamples, method 200 may be triggered responsive to engine repair orservicing (as indicated by a disconnected battery, or input from adiagnostic tool, or input from a GUI).

At 204, method 200 includes selecting a desired compression ratio foroperating the engine based on the estimated engine operating conditions.The engine may be configured with a VCR mechanism (e.g., VCR mechanism194 of FIG. 1) that mechanically alters the engine compression ratiobetween a first, lower and a second, higher compression ratio setting.The VCR mechanism may achieve this by mechanically altering a pistonposition within a cylinder. Alternatively, multiple compression ratiosbetween the first and second compression ratio may be possible. Thecontroller may calculate the fuel efficiency at each possiblecompression ratio of the engine at the given driver power demand andselect the compression ratio that provides the highest fuel efficiency.The controller may compare the fuel efficiency at each compression ratioby comparing the brake specific fuel consumption (BSFC) of the engine ateach compression ratio, for example, from a look-up table stored in thecontroller's memory, the look-up table populated during an initialengine calibration based on a prototype engine with substantially thesame CR on each cylinder. The fuel efficiency of the engine at eachcompression ratio may be determined via a table, a map, an algorithm,and/or an equation, each stored as a function of operating conditions(e.g., engine speed, torque, temperature, humidity, inferred fueloctane, etc.), the settings populated during an initial enginecalibration based on a prototype engine. In general, as engine load orBMEP increases, the compression ratio selected may be decreased due totrade-offs between the efficiency benefits of higher CR (which dominateat lower loads) versus the efficiency penalties of knock-limitedcombustion phasing (which dominate at higher loads). Thus a lowercompression ratio is selected at higher engine loads and a highercompression ratio is selected at lower engine loads.

At 206, the method includes retrieving the actual compression ratiosetting of each cylinder at the desired nominal compression ratiosetting. For example, a look-up table such as the table of FIG. 4 may bereferred to so as to determine if the actual CR for the given cylinderis over or under the desired nominal CR setting.

At 208, the method includes calculating a fuel economy (or fuel usage)associated with each cylinder at the retrieved actual compression ratio.For example, if the actual CR of the selected cylinder is higher thanthe desired nominal CR setting, it may be determined that the fueleconomy for that cylinder is degraded at high loads due to additionalspark retard (later knock-limited combustion phasing). At 210, themethod includes determining an overall fuel economy of the engine bysumming the fuel economy for each cylinder.

At 212, the method includes calculating a fuel penalty associated witheach cylinder at the retrieved actual compression ratio. For example, ifthe actual CR of the selected cylinder is higher than the desirednominal CR setting, and the engine is currently operating at high loadwhere it is knock-limited, it may be determined that there is a fuelpenalty for that cylinder which is a function of the difference betweenthe actual CR of the selected cylinder and the desired nominal CRsetting. At 214, the method includes determining an overall fuel penaltyof the engine by summing the fuel penalty for each cylinder.

As one example, the actual CR data may be retrieved from a look-up tablestored in the controller's memory, such as table 400 of FIG. 4. Thelook-up table may be populated with the data immediately after enginemanufacture, replacement, or major service. For example, CR of eachcylinder may be quantified by measuring dimensions of key engine partsduring manufacturing. Alternatively, the CR of each cylinder may bequantified during end-of-line spin testing by measuring cylinderpressure in each cylinder, or by using radio frequency tranceivers ineach cylinder, or by measuring a crank-angle-resolved crankshaftacceleration profile. The known CR of each cylinder may be stored in thecontroller's memory immediately after engine manufacture, and updated ifnecessary by a service technician after engine replacement or majorservice. In doing so, for each cylinder, it may be determined if theactual CR for the given cylinder is over or under the given CR setting.For example, with reference to table 400 of FIG. 4, the actual CR ofcylinder 1 is significantly higher than the expected setting while theactual CR of cylinder 4 is significantly lower than the expectedsetting, and the difference between expected and actual CR varies withnominal CR. When operating the engine at higher loads, the higher thanexpected actual CR of cylinder 1 may cause cylinder 1 to be moreknock-limited than the other cylinders, thus requiring additional sparkretard. This results in a fuel efficiency penalty for cylinder 1 at highloads.

At 216, the overall fuel penalty due to actual versus nominal CR may becompared to a threshold. If the penalty is lower than a threshold, thatis if there is no significant fuel penalty associated with actual versusnominal CR, then at 218, the method includes continuing engine operationat the desired nominal CR setting selected at 204. For example, anengine with little cylinder-to-cylinder variation in actual CR wouldoperate most efficiently with the desired nominal CR selected at 204because this desired nominal CR was determined from testing of aprototype engine with little cylinder-to-cylinder variation in actual CR(see for example, plot 702 of map 700 of FIG. 7).

Else, if there is a significant fuel economy penalty associated withactual versus nominal CR, then at 220, the method includes actuating theVCR mechanism to a lower CR setting. For example, the controller maysend a signal to the VCR actuator to move the VCR mechanism to reduce CRby 0.2 ratios before repeating the sequence of method 200. For an enginewhere one or more cylinders has a higher than nominal CR, which isoperating at a high load where the engine is knock-limited, the optimumCR will be lower (as shown at plot 704 of map 700 of FIG. 7) than thedesired nominal CR which was determined from testing of a prototypeengine with little cylinder-to-cylinder variation in actual CR.

From each of 218 and 220, the method moves to 222 to determine if enginedilution control is required. In one example, engine dilution control isrequired when engine load is less than a threshold load where combustionstability is a constraint on EGR and/or VCT (or VVL etc.) schedule. Inanother example, engine dilution control is required below a thresholdload which varies with engine speed, temperature, or other factors. Ifdilution control is not required, then at 224, a nominal VCT and/or EGRschedule may be maintained. Else, if dilution control is required, thenat 228, the nominal VCT and/or EGR schedule may be updated. Inparticular, the controller may modify the nominal EGR/VCT schedule basedon the lowest CR of all the cylinders, as determined at 206. The lowerCR leads to degraded combustion stability at light loads, which reducesdilution tolerance and therefore lowers the optimum EGR rate. It alsomoves the optimum VCT/VVL schedule towards lower “internal EGR” (loweroverlap and/or earlier exhaust valve closing time) and/or towards highereffective CR (earlier intake valve closing time). Mapping data from aprototype engine with little cylinder-to-cylinder variation in actual CRmay be used to quantify the optimum (combustion stability limited) EGRand/or VCT schedules as a function of CR. The combustion stability limitis determined by the “worst case” cylinder, in this case the cylinderwith the lowest CR. Therefore the combustion stability limited EGRand/or VCT settings at light loads are simply determined by using the CRof the lowest CR cylinder, instead of using the nominal CR. For example,for an engine where one or more cylinders has a lower than nominal CR,which is operating below a load threshold where combustion stability isa constraint, a lower EGR amount and/or a lower valve overlap and/or andearlier EVC setting may be applied (as shown at plots 506 and 508 ofFIG. 5).

As elaborated at FIG. 3, the EGR and/or VCT schedules may be modifiedany time the VCR mechanism is degraded, such as may occur due tocomponent degradation or due to entry conditions not being met. Inparticular, it may be determined if the actual VCR mechanism position isdifferent from a desired VCR. The unmet VCR actuator entry conditionsmay include conditions pertaining to temperature, oil pressure,electrical current limit, etc. If VCR degradation is detected then alower EGR amount and/or a lower valve overlap and/or and earlier EVCsetting may be applied, as illustrated in FIG. 5.

Turning now to FIG. 3, another example method 300 for calibrating a VCRengine is shown. At 302, as at 202, method 300 includes estimatingand/or measuring engine operating conditions. Engine operatingconditions may include, for example, driver power demand (for example,as based on an output of a pedal position sensor coupled to an operatorpedal); ambient temperature, pressure, and humidity; engine speed,engine temperature; manifold pressure (MAP); manifold air flow (MAF);catalyst temperature; intake temperature; boost level; fuel octane offuel available in a fuel tank; etc.

It will be appreciated that in some cases, method 300 may be triggeredduring a first engine start following manufacture of the engine to allowfor the engine to be calibrated. In other cases, method 300 may betriggered responsive to engine repair or servicing (as indicated forexample by a disconnected battery, or input from a diagnostic tool, orinput from a GUI).

At 304, method 300 includes quantifying the actual CR of each cylinderof the engine at each CR setting of the engine. As one example, theactual CR data may be retrieved from a look-up table stored in thecontroller's memory, such as table 400 of FIG. 4. The look-up table maybe populated with the data immediately after engine manufacture,replacement, or major service. For example, CR of each cylinder may bequantified during end-of-line spin testing, based on measured cylinderpressure, or using radio frequency tranceivers in each cylinder, orbased on measured crank-angle-resolved crankshaft acceleration profile,or based on measured dimensions of key engine parts. The known CR ofeach cylinder could be stored in the controller's memory immediatelyafter engine manufacture, and updated if necessary by a servicetechnician after engine replacement or major service. In doing so, foreach cylinder, it may be determined if the actual CR for the givencylinder is over or under the given CR setting. For example, withreference to table 400 of FIG. 4, the actual CR of cylinder 1 issignificantly higher than the expected setting while the actual CR ofcylinder 4 is significantly lower than the expected setting, and thedifference between expected and actual CR varies with nominal CR. Whenoperating the engine at higher loads, the higher than expected actual CRof cylinder 1 may cause cylinder 1 to be more knock-limited than theother cylinders, thus requiring additional spark retard. This results ina fuel efficiency penalty.

At 306, method 300 includes quantifying fuel flow and maximumin-cylinder mean effective pressure (IMEP) of each cylinder as afunction of the VCR mechanism setting (nominal CR) at the currentoperating conditions. For each possible setting of nominal CR, thecontroller calculates the fuel flow and IMEP for each individualcylinder at its actual compression ratio. The controller may calculatethe fuel flow and IMEP at each compression ratio by using a look-uptable stored in the controller's memory, the look-up table populatedduring an initial engine calibration based on a prototype engine withsubstantially the same CR on each cylinder. The fuel flow per cylinderon the prototype engine is simply the total fuel flow divided by thenumber of cylinders. The IMEP is based on cylinder pressure data on theprototype engine. At 306 the IMEP and the fuel flow for each cylinder iscalculated for each nominal CR (VCR mechanism setting), but using theactual CR of each cylinder. If an engine has little cylinder-to-cylindervariation in CR, then the calculated IMEP and fuel flow in each cylinderis almost the same, and the minimum total engine IMEP and fuel flow areachieved at the same CR as the minimum fuel flow and IMEP for theprototype engine. However, for an engine with high cylinder-to-cylindervariation in CR (such as the engine illustrated in FIG. 4), whenoperated at high loads where the engine is knock-limited, the calculatedfuel flow and IMEP will be different in each cylinder. For cylinder 1 ofthe engine illustrated in FIG. 4, the CR used in the fuel flowcalculation will be higher and IMEP will be lower at higher CR, due toknock-limited combustion phasing. The effects of knock-limitedcombustion phasing are non-linear so optimum nominal CR cannot bedetermined by simply averaging the CR of all cylinders; the cylinderwith the highest CR has a disproportionate effect under knock-limitedconditions. At high loads, the optimum nominal CR for this engine willbe lower than the prototype engine, as illustrated by the dashed line inFIG. 7.

It will be appreciated that while the method suggests quantifying fuelflow and IMEP of each cylinder, this is not meant to be limiting, andthat in alternate examples, other cylinder parameters indicative of fueleconomy and performance may be used. For example, in alternate examples,the metric quantified by the controller (at part load conditions, suchas when below a threshold load) may include efficiency or Brake SpecificFuel Consumption (BSFC), while the metric quantified at higher loadconditions (such as near peak load or when above the threshold load) mayinclude torque, power, or Brake Specific Air Consumption (B SAC).

At 310, it may be determined if the driver demand is higher than athreshold. The threshold may be based on an accelerator pedal position.As such, above the threshold, it may be inferred that the driverprioritizes performance over efficiency, while below the threshold,maximum performance is not required and VCR control can be optimized forefficiency.

If the driver demand is below the threshold, then at 312, the methodincludes selecting the VCR setting corresponding to the minimum totalengine fuel flow. By selecting the VCR setting corresponding to theminimum total engine fuel flow during part load conditions, fuelconsumption and CO₂ emissions can be minimized. Else, at 314, if thedriver demand is above the threshold load, the method includes selectingthe VCR setting corresponding to the maximum total IMEP. By selectingthe VCR setting corresponding to the maximum total IMEP during peak loadconditions, vehicle acceleration performance can be maximized.

From each of 312 and 314, the method moves to 316 to determine if enginedilution control is required. In one example, engine dilution control isrequired when engine load is less than the threshold load. If dilutioncontrol is required, then at 320, the nominal VCT and/or EGR schedulemay be updated. In particular, the controller may modify the nominalEGR/VCT schedule based on the lowest CR of all the cylinders. The lowerCR leads to degraded combustion stability at light loads, which reducesdilution tolerance and therefore lowers the optimum EGR rate. It alsomoves the optimum VCT schedule towards lower “internal EGR” (loweroverlap and/or earlier exhaust valve closing time) and/or towards highereffective CR (earlier intake valve closing time). Mapping data from aprototype engine with little cylinder-to-cylinder variation in actual CRmay be used to quantify the optimum (combustion stability limited) EGRand/or VCT schedules as a function of CR. The combustion stability limitis determined by the “worst case” cylinder, in this case the cylinderwith the lowest CR. Therefore the combustion stability limited EGRand/or VCT settings at light loads are calculated based on the lowest CRcylinder, instead of using the nominal CR. For example, for an enginewhere one or more cylinders has a lower than nominal CR, which isoperating below a load threshold where combustion stability is aconstraint, a lower EGR amount and/or a lower valve overlap and/or andearlier EVC setting may be applied, as illustrated in FIG. 5 which isfurther described below.

If engine dilution control is not required, then at 318, it may bedetermined if there is any degradation or malfunction of the VCRmechanism. In particular, it may be determined if the actual VCRmechanism position is different from a desired VCR. VCR degradation maybe determined responsive to VCR component degradation or due to VCRactuator entry conditions not being met. The unmet VCR actuator entryconditions may include conditions pertaining to temperature, oilpressure, electrical current limit, etc. If VCR degradation is detectedthen a lower EGR amount and/or a lower valve overlap and/or and earlierEVC setting may be applied, as illustrated in FIG. 5.

If VCR degradation is not confirmed, then at 322, a nominal VCT and/orEGR schedule may be maintained. The nominal EGR and/or VCT schedule maybe based on engine operating conditions. Else, if VCR degradation isdetermined, the method returns to 320 to operate with the modified EGRand/or VCT schedule.

FIG. 5 illustrates the effective EGR schedule for a load (BMEP) sweep atvarious CR, based on data from the prototype engine with substantiallythe same CR on each cylinder. Plot 504 shows the EGR schedule with 12:1CR, plot 506 shows the EGR schedule with 10:1 CR, and plot 508 shows theEGR schedule with 8:1 CR. It is understood that the EGR schedule mayalso be a function of engine speed, engine temperature, air temperatureand humidity, etc. The EGR schedule has maximum EGR rates atintermediate BMEP. At higher BMEP the EGR rate may decrease due to EGR'snegative effects on volumetric efficiency. At lower BMEP the EGR ratemay decrease due to EGR's negative effects on combustion stability. Butcombustion stability is also degraded with lower CR. For example, a loadthreshold 502 is shown below which combustion stability is degraded.Therefore lower EGR rates are used when the CR of one or more cylindersis lower. Mapping data from the prototype engine with substantially thesame CR on each cylinder is used to create the table, but the cylinderwith the lowest actual CR is used with the table to determine thecurrent desired EGR rate. Similar methods would be used to limit“internal EGR” by changing the schedule used for VCT, VVL, etc. as afunction of the cylinder with the lowest CR. FIG. 6 illustrates trendsin fuel consumption versus load (BMEP) for various CRs. These trends arewell-known to experts in the field, because they result from fundamentaltrade-offs between the efficiency benefits of higher CR versus theefficiency penalties of knock-limited combustion phasing. The results ofthese trade-offs determine what CR is optimum for each BMEP (the optimumCR also varies with engine speed, fuel octane, inlet air temperature,humidity, etc.). The efficiency benefits of higher CR dominate at lowerBMEP, while the efficiency penalties of knock-limited combustion phasingdominate at higher BMEP, as shown by load limited plot 604. Thus, at lowBMEP where the engine is not knock-limited the optimum CR is high andFIG. 6 shows that the lowest fuel consumption occurs with the highest CRof 13:1. Plots 602 a-h shows fuel consumption decreasing with increasingCR. At high BMEP where the engine is the most knock-limited, the optimumCR is low and FIG. 6 shows that the lowest fuel consumption occurs withthe lowest CR of 8:1. At intermediate BMEP, the two factors trade-off tovarying degrees and the lowest fuel flow occurs at various CR betweenlow and high. The trends shown in FIG. 6 are quantified from testing ofa prototype engine and are used to determine an optimum or desirednominal CR as a function of BMEP, engine speed, fuel octane, inlet airtemperature, humidity, etc. which is used for example in step 204 ofFIG. 2 and step 306 of FIG. 3.

Using the methods described above, cylinder-to-cylinder variations incompression ratio may be better detected and accounted for. By learningfuel flow and IMEP of all cylinders as a function of each nominaldesired CR setting of a VCR engine, actual CR variations of the enginemay be learned, and distinguished from CR data acquired on a prototypeengine. Further, the VCR engine can be reliably calibrated in a morecost effective manner, while relying on existing sensors and actuators.By adjusting the CR setting to provide the highest fuel economy at lowload conditions and highest engine output at high load conditions,engine performance can be improved despite the cylinder-to-cylindervariations in CR. By also adjusting an EGR and VCT schedule of the VCRengine based on the mapped CR of all the engine cylinders, dilutioncontrol is improved, enabling the engine to operate closer to thecombustion stability limit, with fewer NVH issues. Overall, engineperformance and fuel efficiency is increased by improving thecalibration of the VCR engine.

One example method for an engine comprises actuating a variablecompression ratio mechanism of an engine to mechanically adjust a targetcompression ratio of the engine in accordance with an updatedcalibration, the updated calibration based on each of fuel flow and peaktorque of each cylinder at each compression ratio setting of themechanism. In the preceding example, additionally or optionally, theupdated calibration includes: estimating the fuel flow and the peaktorque of each cylinder at a plurality of compression ratio settings;quantifying a total engine fuel flow of the engine at each of theplurality of compression ratio settings as a sum of the fuel flow ofeach cylinder at a corresponding compression ratio setting; and learninga total torque of the engine at each of the plurality of compressionratio settings as a sum of the torque of each cylinder at thecorresponding compression ratio setting. In any or all of the precedingexamples, additionally or optionally, the actuating includes: whendriver torque demand is lower than a threshold, actuating the mechanismto one of a plurality of compression ratio settings having a lowesttotal engine fuel flow; and when driver torque demand is higher than thethreshold, actuating the mechanism to another of the plurality ofcompression ratio settings having a highest total engine torque. In anyor all of the preceding examples, additionally or optionally, thethreshold is based on one or more of accelerator pedal position, enginespeed, fuel octane, ambient temperature, and ambient humidity. In any orall of the preceding examples, additionally or optionally, the methodfurther comprises adjusting an engine dilution calibration based on alowest of a plurality of compression ratios of individual cylinders. Inany or all of the preceding examples, additionally or optionally, theadjusting the engine dilution includes, when the engine load is lowerthan a threshold, using less dilution as the lowest of the plurality ofcompression ratios decreases. In any or all of the preceding examples,additionally or optionally, the method further comprises selecting thetarget compression ratio of the engine based in accordance with anominal calibration. In any or all of the preceding examples,additionally or optionally, the engine is coupled in a vehicle, themethod further comprising updating the nominal calibration of the engineresponsive to engine operation following vehicle manufacture, thenominal calibration based on engine testing prior to the vehiclemanufacture, the vehicle including a hybrid electric vehicle.

Another example method comprises: comparing a target compression ratiofor an engine with an actual compression ratio of each cylinder of theengine; computing a fuel penalty associated with the target compressionratio based on an aggregated difference between the actual compressionratio of each cylinder of the engine and the target compression ratio;and transitioning to a lower compression ratio if the fuel penaltyexceeds a threshold. In the preceding example, additionally oroptionally, each of the target compression ratio and the actualcompression ratio are for a defined compression ratio setting of avariable compression ratio mechanism, the defined compression ratiosetting being one of a plurality of compression ratio settings of theengine. In any or all of the preceding examples, additionally oroptionally, the transitioning includes actuating a variable compressionratio mechanism to mechanically alter the actual compression ratio ofeach cylinder of the engine. In any or all of the preceding examples,additionally or optionally, the method further comprises adjustingengine dilution responsive to the transitioning, the adjustingincluding, when engine load is lower than a threshold, comparing theactual compression ratio of each cylinder of the engine, and applying anengine dilution setting corresponding to a lowest of the actualcompression ratio of each cylinder. In any or all of the precedingexamples, additionally or optionally, adjusting engine dilution includesadjusting one of an exhaust gas recirculation (EGR) amount and avariable cam timing schedule. In any or all of the preceding examples,additionally or optionally, one of a lower EGR amount, a lower valveoverlap, and an earlier exhaust valve closing timing is applied as thelowest of the actual compression ratio of each cylinder decreases.

An example engine system comprises: an engine including a plurality ofcylinders; a VCR mechanism coupled to a piston of each cylinder of theplurality of cylinder for applying one of a plurality of compressionratio settings in a given cylinder via mechanical alteration of a pistonposition within the given cylinder; an EGR passage including an EGRvalve for recirculating exhaust gas from an engine exhaust to an engineintake; and a controller with computer readable instructions stored onnon-transitory memory for: updating a compression ratio calibration ofthe engine based on each of fuel flow and peak torque of each cylinderat each of the plurality of compression ratio settings; and actuatingthe variable compression ratio mechanism of an engine to mechanicallyadjust a target compression ratio of the engine in accordance with anupdated calibration, the updated calibration based on each of fuel flowand peak torque of each cylinder at each compression ratio setting ofthe mechanism. In the preceding example, additionally or optionally, theupdating includes estimating the fuel flow and the peak torque of eachof the plurality of cylinders at each of the plurality of compressionratio settings; for each cylinder, quantifying a total engine fuel flowas a sum of the fuel flow at each of the plurality of compression ratiosettings; and learning a total torque of the engine at each of theplurality of compression ratio settings as a sum of the torque of eachof the plurality of cylinders at a corresponding one of the plurality ofcompression ratio settings. In any or all of the preceding examples,additionally or optionally, the controller includes further instructionsfor: when driver torque demand is lower than a threshold, actuating themechanism to one of the plurality of compression ratio settings having alowest total engine fuel flow; and when driver torque demand is higherthan the threshold, actuating the mechanism to another of the pluralityof compression ratio settings having a highest total engine torque. Inany or all of the preceding examples, additionally or optionally, theupdating is from a nominal compression ratio calibration based on enginetesting prior to vehicle manufacture. In any or all of the precedingexamples, additionally or optionally, the vehicle is a hybrid electricvehicle and wherein the updating is responsive to engine operationfollowing the vehicle manufacture. In any or all of the precedingexamples, additionally or optionally, the controller includes furtherinstructions for: updating an EGR calibration of the engine based on alowest compression ratio setting of one of the plurality of enginecylinders in the updated compression ratio calibration; and actuatingthe EGR valve based on engine load and further based on the updated EGRcalibration.

Another example method for an engine, comprises calibrating acompression ratio schedule of a variable compression ratio engine basedon each of fuel flow and peak torque of each cylinder at eachcompression ratio setting of the engine; and adjusting exhaust gasrecirculation (EGR) flow to the engine in accordance with an updated EGRcalibration schedule based on the compression ratio schedulecalibration. In the preceding example, additionally or optionally, thecalibrating includes, for each cylinder, learning a difference betweenactual compression ratio and commanded compression ratio at each of aplurality of compression ratio settings. In any or all of the precedingexamples, additionally or optionally, the adjusting includes:identifying an engine cylinder having a highest difference betweenactual compression ratio and commanded compression ratio, wherein theactual compression ratio is lower than the commanded compression ratio;and adjusting an EGR flow to the engine based on the actual compressionratio of the identified engine cylinder. In any or all of the precedingexamples, additionally or optionally, the adjusting includes:identifying an engine cylinder having a lowest actual compression ratio;and adjusting an EGR flow to the engine based on the actual compressionratio of the identified engine cylinder. In any or all of the precedingexamples, additionally or optionally, the adjusting in accordance withan updated EGR calibration schedule includes adjusting based on a lowestof a plurality of compression ratio settings of an individual cylinderof the engine. In any or all of the preceding examples, additionally oroptionally, the adjusting further includes, when engine load is lowerthan a threshold load, reducing an EGR flow as the lowest of a pluralityof compression ratio settings decreases, reducing the EGR flow includingone or more of reducing an opening of an EGR valve, adjusting a cylindervalve timing to advance exhaust valve closing, and adjusting thecylinder valve timing to reduce positive intake to exhaust valveoverlap. In any or all of the preceding examples, additionally oroptionally, the engine is coupled in a vehicle, and wherein the updatedEGR calibration schedule is updated from a nominal EGR calibrationschedule based on engine testing prior to vehicle manufacture, thevehicle including a hybrid electric vehicle. In any or all of thepreceding examples, additionally or optionally, the adjusting furtherincludes, when engine load is higher than the threshold load, adjustingthe EGR flow to the engine in accordance with the default EGRcalibration schedule. In any or all of the preceding examples,additionally or optionally, the compression ratio schedule calibrationincludes: estimating each of a fuel flow and a peak torque of eachcylinder at each of a plurality of compression ratio settings of theengine; learning a first parameter indicative of fuel flow of the engineat each of the plurality of compression ratio settings; learning asecond, different parameter indicative of torque of the engine at eachof the plurality of compression ratio settings; and actuating a variablecompression ratio mechanism of each engine cylinder based on a selectionof one of the first and second parameter, the selection based on drivertorque demand. In any or all of the preceding examples, additionally oroptionally, the selection includes selecting the first parameter and notthe second parameter when the driver torque demand is below a threshold,and selecting the second parameter and not the first parameter when thedriver torque demand is above the threshold, the first parameterincluding one of total engine fuel flow and total brake specific fuelconsumption, and the second parameter including one of total enginetorque and total in-cylinder mean effective pressure. In any or all ofthe preceding examples, additionally or optionally, the actuating basedon the selection includes actuating the variable compression ratiomechanism based on an engine cylinder having a lowest value of the firstparameter when the driver torque demand is below a threshold, andactuating the variable compression ratio mechanism based on the enginecylinder having a highest value of the second parameter when the drivertorque demand is above the threshold.

Still another example method comprises: comparing a commandedcompression ratio with an actual compression ratio for each cylinder ofa variable compression ratio engine having a plurality of compressionratio settings; and adjusting exhaust gas recirculation (EGR) flow tothe engine based on the actual compression ratio of an engine cylinderhaving a lowest actual compression ratio. In the preceding example,additionally or optionally, the EGR flow to the engine is reduced from anominal EGR flow as a value of the lowest actual compression ratiodecreases. In any or all of the preceding examples, additionally oroptionally, the adjusting the EGR flow is responsive to engine loadlower than a threshold load, the method further comprising, maintainingthe nominal EGR flow responsive to engine load higher than the thresholdload. In any or all of the preceding examples, additionally oroptionally, reducing the EGR flow includes one or more of reducing anopening of an EGR valve, changing valve timing to decrease positivevalve overlap, and advancing exhaust valve timing to an earlier exhaustvalve closing timing. In any or all of the preceding examples,additionally or optionally, the method further comprises, when engineload is higher than a threshold load, responsive to a difference betweenthe commanded compression ratio and the actual compression ratio of anengine cylinder being higher than a threshold difference, transitioningto a lower compression ratio via mechanical actuation of a variablecompression ratio mechanism.

Another example engine system comprises: an engine including a pluralityof cylinders; a VCR mechanism coupled to a piston of each cylinder ofthe plurality of cylinders for applying one of a plurality ofcompression ratio settings in a given cylinder via mechanical alterationof a piston position within the given cylinder; an EGR passage includingan EGR valve for recirculating exhaust gas from an engine exhaust to anengine intake; and a controller with computer readable instructionsstored on non-transitory memory for: updating a nominal compressionratio calibration of the engine based on each of fuel flow and peaktorque of each cylinder at each of the plurality of compression ratiosettings; adjusting EGR flow to the engine based on the nominalcompression ratio calibration at higher than threshold engine load; andadjusting the EGR flow to the engine based on a lowest compression ratioof the updated compression ratio calibration at lower than thresholdengine load. In the preceding example, additionally or optionally, theadjusting the EGR flow based on the lowest compression ratio includes:identifying one of the plurality of cylinders having a lowest actualcompression ratio; estimating engine dilution for the identified one ofthe plurality of cylinders; and adjusting an opening of the EGR valvebased on the estimated engine dilution. In any or all of the precedingexamples, additionally or optionally, the adjusting the EGR flow basedon the lowest compression ratio includes reducing an opening of the EGRvalve as the lowest compression ratio decreases. In any or all of thepreceding examples, additionally or optionally, the engine is coupled ina hybrid electric vehicle and wherein each of the nominal compressionratio calibration and the nominal EGR calibration are based on enginetesting data gathered prior to vehicle manufacture.

Note that the example control and estimation routines included hereincan be used with various engine and/or vehicle system configurations.The control methods and routines disclosed herein may be stored asexecutable instructions in non-transitory memory and may be carried outby the control system including the controller in combination with thevarious sensors, actuators, and other engine hardware. The specificroutines described herein may represent one or more of any number ofprocessing strategies such as event-driven, interrupt-driven,multi-tasking, multi-threading, and the like. As such, various actions,operations, and/or functions illustrated may be performed in thesequence illustrated, in parallel, or in some cases omitted. Likewise,the order of processing is not necessarily required to achieve thefeatures and advantages of the example embodiments described herein, butis provided for ease of illustration and description. One or more of theillustrated actions, operations and/or functions may be repeatedlyperformed depending on the particular strategy being used. Further, thedescribed actions, operations and/or functions may graphically representcode to be programmed into non-transitory memory of the computerreadable storage medium in the engine control system, where thedescribed actions are carried out by executing the instructions in asystem including the various engine hardware components in combinationwith the electronic controller.

It will be appreciated that the configurations and routines disclosedherein are exemplary in nature, and that these specific embodiments arenot to be considered in a limiting sense, because numerous variationsare possible. For example, the above technology can be applied to V-6,I-4, I-6, V-12, opposed 4, and other engine types. The subject matter ofthe present disclosure includes all novel and non-obvious combinationsand sub-combinations of the various systems and configurations, andother features, functions, and/or properties disclosed herein.

The following claims particularly point out certain combinations andsub-combinations regarded as novel and non-obvious. These claims mayrefer to “an” element or “a first” element or the equivalent thereof.Such claims should be understood to include incorporation of one or moresuch elements, neither requiring nor excluding two or more suchelements. Other combinations and sub-combinations of the disclosedfeatures, functions, elements, and/or properties may be claimed throughamendment of the present claims or through presentation of new claims inthis or a related application. Such claims, whether broader, narrower,equal, or different in scope to the original claims, also are regardedas included within the subject matter of the present disclosure.

The invention claimed is:
 1. A method for an engine, comprising:actuating a variable compression ratio mechanism of an engine tomechanically adjust a target compression ratio of the engine inaccordance with an updated calibration, the updated calibration based oneach of fuel flow and peak torque of each cylinder at each compressionratio setting of the mechanism.
 2. The method of claim 1, wherein theupdated calibration includes: estimating the fuel flow and the peaktorque of each cylinder at a plurality of compression ratio settings;quantifying a total engine fuel flow of the engine at each of theplurality of compression ratio settings as a sum of the fuel flow ofeach cylinder at a corresponding compression ratio setting; and learninga total torque of the engine at each of the plurality of compressionratio settings as a sum of the torque of each cylinder at thecorresponding compression ratio setting.
 3. The method of claim 1,wherein the actuating includes: when driver torque demand is lower thana threshold, actuating the mechanism to one of a plurality ofcompression ratio settings having a lowest total engine fuel flow; andwhen driver torque demand is higher than the threshold, actuating themechanism to another of the plurality of compression ratio settingshaving a highest total engine torque.
 4. The method of claim 3, whereinthe threshold is based on one or more of accelerator pedal position,engine speed, fuel octane, ambient temperature, and ambient humidity. 5.The method of claim 1, further comprising, adjusting an engine dilutioncalibration based on a lowest of a plurality of compression ratios ofindividual cylinders.
 6. The method of claim 5, wherein the adjustingthe engine dilution includes, when the engine load is lower than athreshold, using less dilution as the lowest of the plurality ofcompression ratios decreases.
 7. The method of claim 1, furthercomprising, selecting the target compression ratio of the engine basedin accordance with a nominal calibration.
 8. The method of claim 1,wherein the engine is coupled in a vehicle, the method furthercomprising updating a nominal calibration of the engine responsive toengine operation following vehicle manufacture, the nominal calibrationbased on engine testing prior to the vehicle manufacture, the vehicleincluding a hybrid electric vehicle.
 9. A method, comprising: comparinga target compression ratio for an engine with an actual compressionratio of each cylinder of the engine; computing a fuel penaltyassociated with the target compression ratio based on an aggregateddifference between the actual compression ratio of each cylinder of theengine and the target compression ratio; and transitioning to a lowercompression ratio if the fuel penalty exceeds a threshold.
 10. Themethod of claim 9, wherein each of the target compression ratio and theactual compression ratio are for a defined compression ratio setting ofa variable compression ratio mechanism, the defined compression ratiosetting being one of a plurality of compression ratio settings of theengine.
 11. The method of claim 9, wherein the transitioning includesactuating a variable compression ratio mechanism to mechanically alterthe actual compression ratio of each cylinder of the engine.
 12. Themethod of claim 9, further comprising adjusting engine dilutionresponsive to the transitioning, the adjusting including, when engineload is lower than a threshold, comparing the actual compression ratioof each cylinder of the engine, and applying an engine dilution settingcorresponding to a lowest of the actual compression ratio of eachcylinder.
 13. The method of claim 12, wherein adjusting engine dilutionincludes adjusting one of an exhaust gas recirculation (EGR) amount anda variable cam timing schedule.
 14. The method of claim 13, wherein oneof a lower EGR amount, a lower valve overlap, and an earlier exhaustvalve closing timing is applied as the lowest of the actual compressionratio of each cylinder decreases.
 15. An engine system, comprising: anengine including a plurality of cylinders; a variable compression ratio(VCR) mechanism coupled to a piston of each cylinder of the plurality ofcylinder for applying one of a plurality of compression ratio settingsin a given cylinder via mechanical alteration of a piston positionwithin the given cylinder; an exhaust gas recirculation (EGR) passageincluding an EGR valve for recirculating exhaust gas from an engineexhaust to an engine intake; and a controller with computer readableinstructions stored on non-transitory memory for: updating a compressionratio calibration of the engine based on each of fuel flow and peaktorque of each cylinder at each of the plurality of compression ratiosettings; and actuating the variable compression ratio mechanism of anengine to mechanically adjust a target compression ratio of the enginein accordance with an updated calibration, the updated calibration basedon each of fuel flow and peak torque of each cylinder at eachcompression ratio setting of the mechanism.
 16. The system of claim 15,wherein the updating includes: estimating the fuel flow and the peaktorque of each of the plurality of cylinders at each of the plurality ofcompression ratio settings; for each cylinder, quantifying a totalengine fuel flow as a sum of the fuel flow at each of the plurality ofcompression ratio settings; and learning a total torque of the engine ateach of the plurality of compression ratio settings as a sum of thetorque of each of the plurality of cylinders at a corresponding one ofthe plurality of compression ratio settings.
 17. The system of claim 16,wherein the controller includes further instructions for: when drivertorque demand is lower than a threshold, actuating the mechanism to oneof the plurality of compression ratio settings having a lowest totalengine fuel flow; and when driver torque demand is higher than thethreshold, actuating the mechanism to another of the plurality ofcompression ratio settings having a highest total engine torque.
 18. Thesystem of claim 15, wherein the updating is from a nominal compressionratio calibration based on engine testing prior to vehicle manufacture.19. The system of claim 18, wherein the vehicle is a hybrid electricvehicle and wherein the updating is responsive to engine operationfollowing the vehicle manufacture.
 20. The system of claim 16, whereinthe controller includes further instructions for: updating an EGRcalibration of the engine based on a lowest compression ratio setting ofone of the plurality of engine cylinders in the updated compressionratio calibration; and actuating the EGR valve based on engine load andfurther based on the updated EGR calibration.